To improve acoustic reproduction systems, digital signal processing (DSP) systems have been used to control processing of input signals to generate drive signals for the acoustic production emitted by a loudspeaker.
One previously known form of acoustic production system comprises an active noise control (ANC) production system. Such a system may be conveniently employed in the passenger compartment of the motor vehicle to compensate for the noise of the power train as well as road noise generated as the vehicle travels. In such a system, a sensed signal is input to an adaptive filter that generates an opposing phase signal that cancels out the sensed primary noise signal.
Another known production system often used in vehicles comprises an audio entertainment reproduction system. Since both the audio entertainment system and an interior active noise cancellation system operate to affect the sounds perceived by passengers in the passenger compartment, it would readily appear desirable to consolidate both systems using the same output transducer, for example, one or more loudspeakers, within the passenger compartment to perform both functions. Moreover, the structural simplicity of DSP components and the ease of functional changeover by software modification, renders the combination of noise cancellation and audio reproduction a most practical combination.
Nevertheless, the use of a single power amplifier to drive the transducers in response to both active noise cancellation signals and audio reproduction signals may overdrive the power amplifier and substantially disrupt the signal being delivered to or emanating from the loudspeakers.
In order to avoid distortion in the amplified signals and acoustic response when signal peaks of the output of the overdriven power amplifier are clipped, it has been known to detect the clipping of the output signal and then limit the input to the amplifier or the gain of the amplifier in order to eliminate the clipping and the resultant distortion. For example, U.S. Pat. Nos. 5,001,440, 5,068,620, 4,048,573 disclose apparatus for detecting and limiting clipping of amplifier outputs. U.S. Pat. No. 4,912,424 also recognized that wideband input signal limitation may be undesirable because all frequencies may be attenuated, although only signals in the bass frequency range generate the clipping output. Thus the patent disclosed a process to limit the bass range and the wide band output separately through a bass limiting loop as well as the volume or treble limiting loop. Moreover, bass boost is generally used in an automotive environment to mask low frequency road and engine noise, making clipping more likely in the audio signal environment of a motor vehicle.
In addition, interior active noise cancellation in motor vehicles involves low frequency or bass range signal generation. In such systems, it has been known to employ a power limiter loop. For example, Digisonix DX Model 47 active noise controller processes a correction signal and alters it with a system modeling characteristics filter, including step sizing updates to the adaptive filter, as well as limiting the secondary noise signal or cancellation output to be introduced into the power amplifier. However, such a system is limited to the active noise cancellation circuit and does not limit the input that may be provided by another signal source input to the power amplifier. Moreover, it will be disadvantageous to limit both signal sources at the same rate, especially since the limitation of an output from an audio entertainment reproduction source may be subjectively more noticeable to the occupants of the motor vehicle than the active noise cancellation output of the transducer.